A lithography apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithography apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithography apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
In an immersion lithography apparatus, a liquid is confined to an immersion space by a liquid confinement structure. The immersion space is between a final optical element of a projection system through which the pattern is imaged and the substrate onto which the pattern is transferred or a support table on which the substrate is held. The liquid may be confined to the immersion space by a fluid seal. The liquid confinement structure may create or use a flow of gas, for example to help in controlling the flow and/or the position of the liquid in the immersion space. The flow of gas may help form the seal to confine the liquid to the immersion space.
Defects in the pattern applied to the substrate are undesirable as they reduce the yield, i.e. the number of usable devices per substrate. Because many patterning steps are required to make a device, even a very low rate of defects per exposure can significantly reduce the yield. Generally there is a gap between the edge of the substrate or other object over which the liquid confinement structure passes and the support table Immersion liquid may leak into this gap. The presence in the gap of immersion liquid can cause difficulties including applying a thermal load on the substrate by evaporation, but also risking getting parts of the apparatus wet. Therefore immersion liquid in the gap is desirably removed from the gap between the substrate or other object and the support table using an extraction system. One way in which the extraction system can remove this immersion liquid is to connect the gap to an underpressure source. In that way any immersion liquid in the gap is drawn out of the gap through the support table and thereby removed from the gap.
A difficulty with using an underpressure source to remove immersion liquid from the gap is that this can induce a large cooling load on the substrate or object due to accelerated evaporation of the immersion liquid resulting from gas flow into the gap. A cooling load on the substrate or object is undesirable as this can lead to imaging errors, particularly overlay errors.